Giving a lift to thin films

If you have ever played with one of
those desk ornaments with the suspended metal balls called a Newton’s cradle,
you know that when you pull back and release one or more balls on one side, an identical
number of balls on the other side moves in reaction to this force. A newly developed
technique that mimics this process holds the promise of changing the way that thin
films are deposited.

Researchers at the Paul Scherrer Institute and Empa are working
with a method called laser-induced forward transfer (LIFT). In LIFT, a layer on
a transparent substrate is ablated by an excimer laser, passed through the substrate
and collected on a second substrate. In the process, the laser moves from spot to
spot, producing a patterned thin film.

“The laser defines the material to be transferred [or] deposited.
If we use a 500 by 500-micron beam, we deposit that size, or if we use a round beam,
we deposit a round pattern,” said Thomas Lippert, head of the materials group
at the institute.

Shadowgraphy

Here’s how it works: A sacrificial layer of triazene polymer
is placed between the substrate and the transfer layer, which converts the laser
energy into mechanical energy while at the same time protecting the transfer layer
from radiation. During the process, films made of a stack of triazene polymer, metal
and, optionally, an electroluminescent polymer are irradiated from the back side
by a pulsed XeCl excimer laser operating at 308 nm with a pulse duration of 30 ns.
The evolving “ablation” generates a laser-triggered pressure jet, which
then punches out and catapults the overlying transfer materials, called flyers,
integrally toward the receiver substrate.

The ablation process was imaged by lateral time-resolved shadowgraphy.
“Shadowgraphy,” Lippert explained, “is more or less microphotography
with backlighting. The difference is that the flash is a few nanoseconds, and we
can see solid objects and changes in the refractive index.”

The upper row shadowgraphy microimages show the time-resolved development of the shock
wave and flyer ejection for a laser fluence of 360 mJ/cm2. The flyer consists of
a layer of 80-nm aluminum coated on top of a 350-nm-thick triazene photopolymer.
The flyer stays stable over quite a long distance of more than 0.3 mm. In the bottom
row, the forward-ejection of the same model system was studied to investigate the
fluence dependence of the generated thrust. Images were taken at a constant delay
time of 800 ns after the laser pulse. Flyer velocity and shock wave shape depend
on the applied laser fluence. The bar in the images corresponds to 300 μm. Courtesy
of Thomas Lippert.
Since the sacrificial polymer release layer protects the transfer
layer from the incident UV irradiation, even highly sensitive biomaterials can be
transferred and deposited. An international research collaboration has demonstrated
that the modified LIFT process can transfer not only sensitive materials but also
living mammalian neuroblast cells. With the aid of an approximately 100-nm-thick
aryltriazene photopolymer film, the cells were deposited precisely onto a biological
substrate, gently enough that the functionality was not impaired, and the cells
started reproducing instantly.

Lippert said that the lasers employed are from Lambda Physik and
Quantel.

The group has started a European Union project called E-LIFT to
test applications for the technology that include organic field-effect transistors/organic
LEDs, sensors/bioprinting, and energy harvestors (piezoelectric and thermoelectric)/smart
radio-frequency ID tags.

The researchers published a paper on their findings in the Jan.
6, 2010, online issue of Journal of Physical Chemistry C.

A rare-gas halide or rare-gas metal vapor laser emitting in the ultraviolet (126 to 558 nm) that operates on electronic transitions of molecules, up to that point diatomic, whose ground state is essentially repulsive. Excitation may be by E-beam or electric discharge. Lasing gases include ArCl, ArF, KrCl, KrF, XeCl and XeF.